Alumina reduction cell



Oct. 28, 1969 J. JOHNSTON 3,475,314

ALUMINA REDUCTION CELL Filed Nov. 17, 1965 INVENTOR.

THOMAS JOSEPH JOHNSTON United States Patent US. Cl. 204-243 7 Claims ABSTRACT OF THE DISCLOSURE An alumina reduction cell forming a chamber of molten aluminum in an inclined position. The chamber of molten aluminum forming a current collecting body maintained under hydrostatic pressure. The inclined chamber of molten aluminum extends through the sidewalls of the cell allowing the rodding of any sludge accumulation during operation of the cell.

The invention relates to electrolytic cells for the reduction of alumina to produce aluminum, and more particularly to an improved cathode current collecting chamber for such cells.

The conventional design of an electrolytic furnace for the reduction of alumina utilizes steel bars imbedded in a carbon lining of the shell as cathode and current conductors. This system has reasonable long term stability but exhibits an appreciable ohmic voltage drop between the bus bars and the liquid aluminum cathode. Temperature fluctuations in the carbon impregnated with fluorides initiates the rupture and deformation of the cell lining.

Another concept of cell lining and electrical connection to the liquid aluminum is represented by the use of a refractory hard metal, e.g., TiBg bars as current conductors either in combination with a carbon bottom or with other refractory bottom materials. While the cathodic potential drop with the use of such refractory hard metal bars is less than that for a steel-carbon combination, the TiB bars have been found subject to thermal shock, and have shown varying degrees of chemical attack by their operating environment.

It has also been proposed to use as a part of the cathode current collecting system, a chamber in the form of a well arranged at the side of the electrolytic cell to receive and hold molten aluminum separated from the aluminum cathode of the cell proper by a refractory wall including one or more bricks of a refractory hard metal such as TiB The cathode current flows from the aluminum pad at the bottom of the cell through the TB; bricks. At this point the molten aluminum in the adjacent well becomes the current conductor which carries the current to hard metal current leads partially immersed in the aluminum in the well. This arrangement suffers from the problems inherent in the exposure of TiB bars to the operating environment of the cell as noted above. Additionally, it gives rise to the problems associated with the accumulation of sludge at the bottom of the cell covering, or partially covering, the exposed surfaces of the TIB2 bricks and interfering with the current flow.

In has been an object of the present invention to provide a cathode current collecting system which eliminates or minimizes the foregoing difiiculties and which yields new results and advantages as will appear.

SUMMARY In accordance with my invention as applied to an alumina reduction cell having refractory sidewalls and a bottom forming a pot to hold the molten contents of the cell, one or more cathode current collecting chambers are provided, such chambers extending on an incline ice from a lower portion of the pot upwardly and outwardly through its refractory sidewall or walls. Such chamber(s) opens directly into the pot at its lower end to hold molten aluminum supported by the hydrostatic pressure of the molten contents of the cell. A cathode current lead is arranged in contact with the aluminum in an upper portion of the chamber. In a preferred form of my invention the cathode current collecting chamber is of generally tubular form and the current lead is arranged to one side of its axis so that a rod can be passed through-the chamber to free any sludge which may accumulate in the region of the inner end of the cathode current collecting chamber. A particular advantage of the inclined chamber, besides facilitating rodding of accumulated sludge, is that it opposes movement of solidified sludge into the current collecting areas. This is occasioned at least in part by the fact that sludge particles will slide down the inclined surface of the cathode chamber under the force of gravity and drop into the bottom of the pot, clear of the opening at the lower end of the chamber. Such opening, while above the bottom of the pot, is below the upper level of the molten aluminum cathode during normal operation of the cell.

DESCRIPTION With reference to the accompanying drawings, the best mode contemplated for carrying out the invention will now be described.

FIG. 1 is a partial vertical sectional view showing an alumina reduction cell embodying the invention.

FIG. 2 is a similar view illustrating a second embodiment of the invention.

The cell itself may be of conventional construction, FIG. 1, having refractory sidewalls 1 and a bottom 2 within a metal shell 3, forming a pot to hold the molten contents of the cell, such contents comprising the electrolyte and aluminum cathode as designated by the legends. It is contemplated that the usual carbon anode will be used, in which will be imbedded suitable current conducting pins 4 serving as connectors for the positive DC. current. The bottom 2 of the pot may be of carbon, as is usual, or of other selected refractory composition such as the cryolite-alumina refractory of the Dewey Patent No. 3,093,570, granted June 11, 1963.

A refractory tube 5 forms a cathode current collecting chamber 6 extending on an incline from a lower portion of the pot as at 7, thence upwardly and outwardly through the refractory sidewall 1. At 7, the chamber 6 opens directly into the pot at its lower end to hold molten aluminum supported by the hydrostatic pressure of the molten contents of the cell, such contents comprising the aluminum cathode and the electrolyte above it. A cathode current lead 8 is arranged in contact with the aluminum in an upper portion of the chamber 6. Tube 5 may advantageously be made of a silicon nitride-bonded silicon carbide. It should extend on an angle from the horizontal of on the order of 50 to depending somewhat on the relationship between the depth of the molten contents of the cell and the thickness of the sidewall 1. This angular disposition of the cathode current collecting chamber facilitates performance of three desirable functions: (1) frozen or solidified particles of cryolite and cryolitealumina muck which normally are pushed along through the aluminum layer by forces connected with the flow of the current are hindered from rising in the current collecting chamber by the force of gravity acting on the particles, (2) in the event of any plugging in the current connection by such particles, the cathode bus connection 8 can be removed so that the chamber 6 can be rodded free of the accumulated sludge, and (3) the inclination of the chamber from the vertical permits direct access to the opening 7 between the current connection and the aluminum cathode and hence facilitates the cleaning operation.

Cathode current lead 8 can advantageously be made of titanium diboride or graphite. Other refractory conductive materials, suitably protected from oxidation, may be used instead. The temperature in the upper end of chamber 6 will be somewhat lower than the temperature at the lower end of the chamber where the chamber is open to the aluminum cathode so that the life of the electrical connectors should be longer than if they were exposed directly to the aluminum cathode as has been the past practice. It is also contemplated that the connectors 8 could be comprised of large copper rods, the size of the rods being such that the heat loss from the rod would maintain at least a thin film of solid aluminum on the surface of the copper thereby protecting the copper from corrosion by the liquid aluminum.

The modified construction shown in FIG. 2 is generally similar to that which has been described With reference to FIG. 1. The dilferences are these: sidewalls 9 are thinner than sidewalls 1. This being so, the upper end of the tube 10 which forms chamber 6 may project somewhat beyond the shell 3 of the pot. If necessary or desired, the projecting upper end of the tube may be insulated by means of a suitable refractory material 11 contained in a box 12 affixed as shown to the upper part of the shell 3 of the pot. This box structure may, if desired, extend around the periphery of the upper edge of the pot so as to accommodate any desired number of cathode current collecting chambers 6. Another feature of the particular construction shown in FIG. 2 is that the tube 10 has a widened portion 13 at its upper end to receive the current lead 8 in a position to one side of the axis of the tube so as to leave an unobstructed passage 14 for rodding of sludge accumulated in the region of the lower end of the chamber, the rod being inserted in the manner indicated by the arrow 15. In the FIG. 1 construction the current leads 8 would be removed as needed to perform the rodding operation. Both constructions furnish means for removing, examining and replacing one or more of the connectors without interrupting the operation of the cell. Also, if desired, the inclined cathode current collecting chambers 6 can be used for siphon action tapping of aluminum from the cell.

The terms and expressions which I have employed are used in a descriptive and not a limiting sense, and I have no intention of excluding equivalents of the invention described and claimed.

I claim:

1. In an alumina reduction cell having refractory sidewalls and a bottom forming a pot adapted to hold the molten contents of the cell, a cathode current collecting chamber forming a portion of said molten contents of said pot at a laterally displaced position from the anode into a current collecting body lying in an incline on the order of 50 to 80 to the horizontal and extending upwardly and outwardly through the sidewall of said pot from a point above the bottom thereof and wherein the hydrostatic pressure of the molten contents of the pot cause the molten metal to rise in said inclined current collecting body above the level of the metal in the pot and a cathode current lead in contact with said molten metal in the upper portion of said body of metal whereby said cathode collecting body of molten metal may be maintained free of sludge.

2. An alumina reduction cell as in claim 1 in which the cathode current collecting chamber is of generally tubular form said chamber opening directly into the pot at its lower end to hold molten aluminum supported by the hydrostatic pressure of the molten contents of the cell, said cathode current lead arranged to one side of the axis of the tubular chamber allowing a rod to be passed through the chamber to free accumulated sludge in the region of the lower end of said cathode current collecting chamber.

3. An alumina reduction cell as in claim 1 in which the cathode current collecting chamber is a refractory tube for connecting directly with said molten contents of said pot.

4. An alumina reduction cell as in claim 1 in which the cathode current collecting chamber is a refractory tube for connecting directly with said molten contents of said pot, and said current lead comprises an electrically conducting refractory body extending into the molten metal near the upper end of said refractory tube.

5. An alumina reduction cell as in claim 1 in which the cathode current lead comprises a copper rod positioned to contact the molten metal, said molten metal being aluminum, said aluminum being in the upper portion of said body of metal formed into a current collecting body, said copper rod being adapted to permit heat loss sufficient to maintain at least a thin film of solid aluminum on the surface of said copper rod thereby protecting the copper rod from corrosion by the liquid aluminum.

6. An aluminum reduction cell having refractory sidewalls and a bottom forming a pot adapted to hold the molten contents of the cell, at least one cathode current collecting chamber extending on an incline of 50 to to the horizontal from a lower portion of the pot upwardly and outwardly through a refractory sidewall thereof to provide a cylindrical body of molten metal that may be maintained free of sludge, said chamber opening directly into the pot at its lower end at a point above the bottom thereof to hold the molten metal at the bottom of the pot in contact with the molten metal, said metal being aluminum, said molten aluminum being supported in said chamber by the hydrostatic pressure of the molten contents of the cell inclusive of the molten electrolyte carried above the molten aluminum cathode, and a cathode current lead arranged in contact with the molten aluminum in the upper portion of the chamber.

7. An alumina reduction cell as in claim 6 in which said chamber is of generally tubular form and has a widened portion at its upper end to receive the current lead in a position which leaves an unobstructed passage for rodding of sludge accumulated in the region of the lower end of the chamber.

References Cited UNITED STATES PATENTS 400,664 4/1889 Hall 204243 XR 510,276 12/1893 Lyte 204247 XR 2,451,490 10/1948 Johnson 204244 XR 3,028,324 4/1962 Ransley 20467 3,321,392 5/1967 McMinn et a1. 204243 3,392,092 7/1968 Diller 20467 FOREIGN PATENTS 616,450 Great Britain.

JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. or. X.R. 20 1-292 

